Well, gearing a multibox group is the same as gearing a single toon.

When tBC was new, nobody was geared, so you could not join the guilds Kara farming runs.

Everyone was gearing up, just treat your team the same.
You run the quests (usually group quests) that are worthwhile in terms of item rewards.
Run the max level non-heroic instances until you have most of the gear from these. Non-heroics can be run as many times a day as you have the time.
Once you've got near the best or even 75% of your slots filled with this gear, try a weaker/easier heroic; see how you do.
If you're lacking, farm a bit more gear from non-heroics. There will be some heroics easier to do then others, for your group composition/playstyle.
Try a few. Once you find something you can do, farm the piss out of it - make sure to run it as close to every day as possible, the badges will add up.
Any time you get a gear upgrade, try something that was a little out of reach, it might be doable now.

Post what worked for you and what did not on the PvE tactics forum.
Others will have insight to what you can try differently.
Others might get something that helps them.
Read how someone else tackles a given pull - it might make the difference for you, or even give you a tactic for a different situation.
Keep experimenting, even on things you've done successfully, there's always room for improvement.

If the game gets frustrating, take a break from it. Remember its a pass-time, and should be played for enjoyment.
Come back at it later on, recharged and ready to go.
If you're persistent, you'll eventually beat each step along the way.
And at that point, the time invested to get there will have been worthwhile.